Process and device for working a workpiece

ABSTRACT

The application relates to a process for working a workpiece, in which process an abrasive liquid is sprayed onto the workpiece, via a nozzle, at relatively low pressures which are sufficient to shape and/or polish the surface of the workpiece. The workpiece can be both shaped and polished in a single working step. Abrasive particles or polishing particles may be contained in the abrasive liquid. The pressure of the abrasive liquid lies below 50 bar, preferably below 20 bar. By arranging two nozzles in such a manner that the liquid jets intersect one another at a point, it is possible to set an accurate working depth.

The invention relates to a process and to a device for working aworkpiece wherein an abrasive liquid is sprayed onto the workpiece, viaat least two nozzles, each of which is disposed at an angle with respectto the workpiece and the liquid jets from which intersect one another onor below the workpiece surface, such as for example for shaping orpolishing optical components.

It is known to form curved optical surfaces in optical materials, suchas quartz or glass, by means of grinding and polishing. In athree-component process of this nature, a tool, such as a mould, is usedto press abrasive particles in a slurry onto the optical surface whichis to be worked. The tool is subjected to load and is moved with respectto the workpiece. Although the known process makes it possible toaccurately work the optical components, it is relatively lengthy.Furthermore, more complex shapes, such as aspherical optical components,cannot easily be formed using the known method.

A process and device according the preamble of claim 1 is known fromDE-A-4407271. In this patent application a process is described for theworking of surfaces at pressures ranging from 600-4000 bar. The highpressure used in the known process results in a relatively largeroughness of the worked surface, such that the known process will not besuitable for shaping or production of for instance an optical component.

Therefore, one object of the present invention is to provide a processand device with which a workpiece can be shaped, ground or polishedaccurately and quickly. A further object of the present invention is toprovide a process and device of this nature with which it is easy toimpart complex shapes to a workpiece, in particular to opticalcomponents made of a refractive optical material, such as quartz, glassor plastic, or of a reflective optical material, such as metals andceramic materials. Yet another object of the present invention is toprovide a process and device which allow the surface to be shaped in asingle operation and to be polished with the desired level of accuracy,for example to a roughness of 1 nanometre RMS or better.

To this end, the process according to the invention is characterized inthat the abrasive liquid is sprayed onto the workpiece at a pressure ofless than 50 bar, preferably of less than 20 bar, to shape and/or polishthe surface of the workpiece. “Abrasive liquid” is in this contextintended to mean a liquid which can be used to grind a surface to arelatively high roughness or to polish it to a lower roughness.

Surprisingly, it has been found that the abrasive liquid provides verycontrolled working of the surface of the workpiece at relatively lowpressures, such as 50 bar or lower. The abrasive liquid, whichpreferably contains abrasive particles, has a low velocity at these lowpressures, so that material is removed in a controlled manner withoutforming irregular pitting in the surface.

At the point where the liquid jets cross or intersect one another, theimpulse of the abrasive particles or polishing particles is reduced tosuch an extent that no further material is removed below this point. Inthis way it is possible to set the working depth very accurately.

It has been found that the process according to the present inventionmakes it possible, when the abrasive liquid used is water containingsilicon carbide particles with a size of approx. 20 μm as the abrasive,to polish a surface of BK7 to an ultimate roughness of 1.5 nm RMS. Aconventional polishing method with a particle size of this natureresults in a roughness of approx. 5 μm.

It should be noted that a device for cutting glass using a high-speedjet of liquid is known per se from American patent U.S. Pat. No.4,787,178. However, the nozzle pressures which are used for cutting theglass are in the order of magnitude of 2000 bar. For this reason, themethod is unsuitable for very accurate surface-working operations.

It is also known from American patent U.S. Pat. No. 5,573,446 to shapeoptical components by moving a stream of gas which contains abrasiveparticles over the surface of the workpiece in a raster pattern. Thisshaping process only provides limited accuracy, so that an opticalcomponent has to be polished separately after it has been shaped.

The process according to the present invention differs from the abovemethods by the fact that material is removed in a very controlledmanner, making it possible, within a short time, both to shape theworkpiece and to polish it until the desired roughness is reached.

The abrasive liquid according to the present invention may comprise anumber of liquids, such as water or an organic liquid, such as octanol.Preferably, abrasive particles or polishing particles are added to anabrasive liquid, such as for example #800 silicon carbide or particleswhich have similar properties. Other suitable abrasive particlescomprise diamond or aluminiun oxide, while diamond or cerium oxide canbe used for polishing. The rate at which material is removed from thesurface of the workpiece depends on the concentration, dimensions andhardness of the abrasive particles and on the type of abrasive liquid,the velocity of the abrasive liquid when it leaves the nozzle, thecontact time, the geometry, the relative dimensions and orientation ofthe nozzle with respect to the workpiece surface, and the like. Theabrasive-liquid pressures employed are preferably less than 50 bar, suchas for example 5 bar. The diameter of the nozzle is preferably smallcompared to the dimensions of the workpiece, such as between 10 cm and0.1 mm, preferably between 1 cm and 0.5 mm, and particularly preferablybetween 5 mm and 0.5 mm. The diameter of the workpiece may, for example,amount to 100 mm.

The operation is relatively insensitive to the distance between nozzleand workpiece.

Although the process according to the invention can be used on amultiplicity of materials, the method is particularly suitable forrefractive optical materials, such as for example silicon, glass,sapphire, quartz, optical plastics, but also for reflective opticalmaterials, such as metal or ceramic materials. Owing to the low energyof the abrasive liquid and the abrasive particles, material is removedgradually without pitting or scratches being formed. During theoperation, one nozzle may be moved with respect to the workpiece, forexample in a raster pattern. It is also possible to employ a series ofnozzles and to rotate the workpiece about its axis of rotation at thesame time. By linking the movement of the nozzle to the movement of theworkpiece, it is possible to grind and polish complex geometric shapes,such as for example toric surfaces. By moving the axis of rotation ofthe workpiece it is possible, for example, to shape and polish a toricsurface. The cross section of the nozzle may be circular, elliptical,triangular or rectangular, or may be in the form of a series of ellipsesor rectangles in order to form a plurality of slots in a singleproduction run, for example in order to form binary optical elements.

A number of aspects of the process and device according to the presentinvention will be explained in more detail with reference to theappended drawing, in which:

FIG. 1 shows a diagrammatic side view of a nozzle and a workpiece foruse in the process according to the present invention,

FIGS. 2a to 2 c show diagrammatic views of a pair of nozzles withintersecting liquid jets,

FIG. 3 shows a method according to the present invention for formingmicrotexturing in a material,

FIG. 4 shows a headstock of a lathe with an integrated tool and nozzle,and

FIG. 5 shows a method of shaping a rotationally symmetrical surface bymeans of pressure variations from a nozzle according to the invention.

As shown in FIG. 1, a nozzle 1 is moved to a distance σ above aworkpiece 2. In this case, the distance σ is a few millimetres, such asfor example 3 mm. The abrasive liquid 3 is sprayed onto the workpiece 2at a pressure of, for example, 5 bar. The abrasive liquid 3 used iswater containing #800 SiC abrasive particles. The diameter Φ is, forexample, 2 mm. In the exemplary embodiment shown, the angle a betweenthe nozzle 1 and the workpiece surface is 90°, and the nozzle 1 isadvanced with respect to the surface of the workpiece 2 in the directionof the arrow and at a velocity V. At the relatively low pressure and thegiven diameter of the nozzle 1, the flow of the abrasive liquid 3 willbe laminar. The rate and level of fineness of the working can beadjusted by varying diameter Φ of the nozzle, the pressure of theabrasive liquid 3, the angle α with respect to the workpiece, thedistance ó between the nozzle 3 and the workpiece 2 and the velocity V.

A test was carried out using a polishing abrasive containing relativelycoarse SiC particles with a dimension of approx. 22 μm in water at aconcentration of 10%. The polishing abrasive was guided, via a nozzle ofcircular cross section with a diameter of between 0.2 and 1.6 mm,towards an optical surface made from planar BK7 glass at pressures ofbetween 0.5 and 6 bar. The surface roughness of the optical surface wasreduced from 350 nm RMS to 25 nm RMS. It was also possible to use thegrinding means to form a polished surface with a surface roughness of1.6 nm RMS without bringing about an increase in the surface roughness.It was found that no polishing or grinding effect was observed atpressures of below 1 bar. During the test, the polishing abrasive wasdeployed in a closed circuit in which used polishing abrasive was reusedafter filtering.

FIG. 2 shows an arrangement in which two nozzles 4, 5 are disposed at anangle β between the nozzle and the normal to the surface, so that theliquid jets 6, 7 intersect one another at a point 8. At this point 8,the impulse of the liquid jets and the abrasive particles will bereduced to such an extent that no material is removed below the level ofplane a of the point 8. This makes it possible to accurately set thedepth to which material is removed. FIG. 2b shows a device in which thetwo nozzles 4 and 5 are attached to a head 10 of a machining device. Thematerial will be removed from the workpiece 11 to a depth a whichcorresponds to the intersection point 8 of the liquid jets 4 and 5 asshown in FIG. 2c. The advantage of the device according to the presentinvention lies in a very accurately defined working depth and a very lowlevel of wear to the tool, and also in the fact that the liquid jetsfrom the nozzles 4 and 5 clean and cool the workpiece during operation.The device described in FIG. 2 can be used to form aspherical opticalcomponents as described in International Patent ApplicationPCT/N196/00343 in the name of the applicant. This device can also beused in a lathe or a precision-grinding machine to replace the diamondhead or the diamond wheel.

FIG. 3 shows how a nozzle 12 according to the present invention can beused to form a micro-optical component 13 in a workpiece 14. Themicro-optical component may, for example, comprise a parabolic mirror.The shape depends on the geometry of the nozzle, the angle α, thevelocity of the abrasive liquid and the velocity with respect to theworkpiece surface. Furthermore, the process and the device according tothe present invention may be used to provide optical components with anidentifying mark by forming small, concave polished points having adepth in the order of a few nanometres. These identifying marks willonly be visible against dark field illumination and can be used foraligning the optical components.

FIG. 4 shows a headstock 15 of a milling cutter, lathe orprecision-grinding machine with a diamond tool 16 and a nozzle 17 forforming an aspherical surface in a workpiece 18. Firstly, the tool 16can be used to form the desired surface shape, after which, in asubsequent or in the same working step, this surface can be polishedusing the nozzle 17.

FIG. 5 shows how a nozzle 20 is moved in the direction of the arrow andat a velocity V over a workpiece 21 which is rotated about axis ofrotation 22. During the movement of the nozzle 20, the pressure P of theabrasive varies in a controlled manner in accordance with the profileindicated in the figure, so that the desired surface shape is obtained.It is also possible to vary the speed of displacement V of the nozzle.

What is claimed is:
 1. Process for working a workpiece, wherein anabrasive liquid is sprayed onto a workpiece, via at least two nozzles,each of which is disposed at an angle with respect to the workpiece andliquid jets from which intersect one another on or below the workpiecesurface, wherein the abrasive liquid is sprayed onto the workpiece at apressure of less than 50 bar.
 2. Process according to claim 1, in whichthe workpiece is both shaped and polished by the abrasive liquid. 3.Process according to claim 1, wherein the abrasive liquid comprisesabrasive particles or polishing particles.
 4. Process according to claim1, wherein the abrasive particles comprise #800 SiC particles. 5.Process according to claim 1, wherein the diameter of the nozzle isbetween 10 cm and 0.1 mm.
 6. Process according to claim 1, wherein thematerial which is to be worked comprises one of an optical material,quartz, metal and a ceramic material.
 7. Process according to claim 1,wherein the nozzle is moved with respect to the workpiece.
 8. Processaccording to claim 7 wherein movement of the nozzle with respect to theworkpiece comprises rotating the workpiece.
 9. Process according toclaim 7 wherein the movement comprises displacing the nozzle. 10.Process according to claim 9, wherein the nozzle is moved in a rasterpattern, parallel to the workpiece.
 11. Process according to claim 1,wherein at least two mutually connected nozzles are used.
 12. Device forworking materials, comprising at least two nozzles which are positionedin such a manner with respect to one another that the liquid jets fromthe nozzles intersect one another at a point, a feed line which isconnected to the nozzle and contains a pump for feeding an abrasiveliquid to the nozzle, wherein the pressure of the feed pump is less than100 bar.
 13. The process of claim 1, wherein the abrasive liquid issprayed onto the workpiece at a pressure of less than 20 bar.
 14. Theprocess of claim 12, wherein the pressure of the feed pump is less than20 bar.
 15. The process of claim 5, wherein the diameter of the nozzleis between 2 cm and 0.5 mm.
 16. The process of claim 15, wherein thediameter of the nozzle is between 2 mm and 0.5 mm.
 17. The process ofclaim 4 wherein the abrasive particles have properties similar toproperties of SiC particle.